Water base ink set for ink-jet recording

ABSTRACT

An ink set for ink-jet recording of the present invention comprises a black ink which has a surface tension of not less than 40 mN/m at 25° C., and a color ink which has a surface tension of not less than 40 mN/m at 25° C. The viscosity ratio of the color ink with respect to the black ink is not less than 1.3. The feathering of the ink is avoided, and the bleeding, which would otherwise appear at the boundary between the black ink and the color ink, is avoided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a water base ink set for ink-jetrecording including a black ink, and an ink-jet recording apparatuswhich accommodates the same.

[0003] 2. Description of the Related Art

[0004] The ink discharge system, which has been hitherto known as theink-jet recording method, includes, for example, the electrostaticattraction method, a method in which mechanical vibration ordisplacement is applied to the ink with a piezoelectric element or thelike, and a method in which bubbles are generated by heating the ink toutilize the pressure generated thereby. Ink droplets are formed by meansof the ink discharge method as described above, and all or a part of theink droplets are adhered to a recording objective such as paper toperform the recording. Those known and used as the ink to be employedfor the ink-jet recording method as described above include those inwhich a variety of water-soluble dyes or water-dispersible pigments aredissolved or dispersed in liquid media each comprising water or acombination of water and water-soluble organic solvent.

[0005] In order to adequately perform the recording for a long period oftime by using the ink as described above, for example, it is necessaryto satisfy the following conditions. That is, the characteristic values(for example, those of the viscosity, the surface tension, and thedensity) of the ink are appropriate values. No deposited matter isgenerated and no physical property value is changed by the heat or thelike (to avoid any clog-up at the nozzle or the orifice of the recordingapparatus and discharge the ink stably). The recorded image isexcellent, for example, in water resistance and light resistance.

[0006] When the recording is performed with an ink-jet printer by usingan ordinary water base ink, exclusive ink-jet paper is sometimes used,in order to obtain good printing qualities without involving anyblurring of the ink. However, in recent years, it is more demanded toperform the recording on the regular paper rather than the recording onthe exclusive ink-jet paper, in view of the running cost and theconsideration of the environment.

[0007] Further, as for the color of the ink, the color ink-jet printeris overwhelmingly demanded as compared with the monochrome ink-jetprinter in the market directed to home use and office use.

[0008] However, when the recording is performed on the regular paper, aproblem arises such that the printing quality is remarkably deterioratedas compared with the case in which the recording is performed on theexclusive ink-jet paper. For example, the following problem arises. Thatis, when the ink permeates into the recording paper, then the inkspreads non-uniformly, and the edge of an image portion is notched,making it impossible to obtain any sharp edge of the image portion. Thisphenomenon is generally called “feathering”.

[0009] Further, in the case of the color ink-jet printer, anotherproblem arises in addition to the problem described above such that theinks are mixed with each other at the portion (hereinafter referred toas “boundary”) at which the different colors are disposed adjacently,and the both inks are blurred, resulting in the deterioration of theprinting quality. This phenomenon is generally called “color bleed”. Thecolor bleed tends to be conspicuous, for example, especially whenletters or lines are recorded with the black ink having a relativelydark color on the background of the color ink, i.e., yellow, magenta,cyan or the like having a relatively bright color.

[0010] Many techniques have been used in order to solve the problems asdescribed above, suppress the feathering and the color bleed, and obtainthe good printing quality.

[0011] At first, in order to reduce the feathering, a method isgenerally used, in which the surface tension of the ink is increased.The higher the surface tension is, the better the successfully obtainedprinting quality is, in which the feathering scarcely appears.

[0012] A technique, which is used to reduce the color bleed, is a methodin which the surface tensions of the color ink and the black ink arelowered so that they are in equivalent degrees (within ±10% of the blackink). In most cases, the surface tension is adjusted to be about 28 to38 mN/m.

[0013] In the case of the ink set having been hitherto used, no specialconsideration is made for the viscosity of the ink. For example, in viewof the discharge stability, the viscosities of the black ink and thecolor ink are adjusted to be in approximately equivalent degrees.

[0014] Unlike the general techniques as described above, it has beensuggested in Japanese Patent Application Laid-open No. 6-106841 that apigment is used for a coloring agent of a black ink, and a precipitatingagent is added to a color ink in order to precipitate the black pigment.This suggestion is based on the following method. That is, the blackpigment is aggregated and precipitated by using a polyvalent metal saltas the precipitating agent dissolved in the color ink, at the boundaryat which the black ink and the color ink make contact with each other.Accordingly, the fluidity of the black pigment in the black ink issuppressed to avoid the color bleed.

[0015] As described above, in order to reduce the feathering and thecolor bleed, the techniques corresponding to the respective cases havebeen hitherto used.

[0016] However, most of the conventional inks as described above haveinvolved the following problem. That is, when the recording is performedon the regular paper, any one or all of the feathering and the colorbleed of the black ink and the color ink are caused. It has beenimpossible to obtain any satisfactory printing quality in which all ofthem are simultaneously improved.

[0017] As described above, there has been the following problem. Thatis, if the surface tension is about 28 to 38 mN/m as in the conventionalink set, and the viscosity is approximately equivalent to one another inratio, then the feathering has been unsatisfactory. If it is intended toincrease only the surface tension, it has been impossible to satisfy thecolor bleed. It has been impossible to simultaneously satisfy therequirements for the color bleed and the feathering.

SUMMARY OF THE INVENTION

[0018] The present invention has been made in order to solve theproblems as described above, an object of which is to provide a waterbase ink set for ink-jet recording which simultaneously satisfy all ofthe requirements for the black ink feathering, the color ink feathering,and the color bleed, even when recording is performed on regular paper,and an ink-jet recording apparatus which accommodates the same.

[0019] According to a first aspect of the present invention, there isprovided an ink set for ink-jet recording, comprising a black inkwhich-has a surface tension of not less than 40 mN/m at 25° C.; and acolor ink which has a surface tension of not less than 40 mN/m at 25°C.; wherein a viscosity ratio of the color ink with respect to the blackink (viscosity of the color ink/viscosity of the black ink) is not lessthan 1.3. The surface tensions of the color ink and the black ink may benot less than 45 mN/m at 25° C.

[0020] According to a second aspect of the present invention, there isprovided an ink-jet recording apparatus comprising an ink-jet head; anink tank which accommodates an ink to be supplied to the ink-jet head;and an ink set of the present invention which is accommodated in the inktank. The ink set may be in a form of ink cartridge. The ink-jetrecording apparatus of the present invention accommodates the ink set ofthe present invention. Therefore, it is possible to simultaneously avoidthe feathering and the bleeding. The ink tank may be an ink containerfixedly provided in the ink-jet recording apparatus or an ink cartridgewhich is replaceable.

[0021] In order to solve the problems involved in the conventionaltechnique, we have made diligent studies. As a result, at first, thefollowing fact has been revealed. That is, in order to satisfy therequirement for the feathering, it is advantageous that the ink has ahigher surface tension. However, in general, the effect to avoid thefeathering is poor unless the surface tension is not less than 40 mN/m.On the contrary, the following fact has been revealed. That is, if theviscosity of the black ink is approximately equivalent to that of thecolor ink, and the surface tension is not less than 40 mN/m, then thepermeation performance of the inks into the recording paper is lowered,and the time of contact between the black ink and the color ink isprolonged on the recording paper. Therefore, the inks are mixed witheach other during the period until the inks permeate into the recordingpaper, resulting in the occurrence of the color bleed.

[0022] On the other hand, in order to avoid the color bleed, if thesurface tensions are lowered, i.e., if the viscosity of the black ink isapproximately equivalent to that of the color ink as in the conventionalink set, and the surface tensions are about 28 to 30 mN/m, then thecolor bleed is reduced to some extent. However, the following problemarises. That is, the permeation performance into the recording paper istoo high. The inks spread non-uniformly along the surface of therecording paper, and the feathering is caused in a deteriorated manner.

[0023] The effect to avoid the color bleed according to the presentinvention may be explained as follows. In the water base color ink setaccording to the present invention, at first, the surface tensions ofthe black ink and the color ink at 25° C. are adjusted to have therelatively high values of not less than 40 mN/m, and thus the black inkand the color ink are prevented from the feathering. Further, theviscosity ratio between the black ink and the color ink at 25° C. isadjusted to be not less than 1.3. Accordingly, the effect to avoid thecolor bleed is obtained. Owing to the effects as described above, it ispossible to simultaneously satisfy the requirement for the color bleedin addition to the requirements for the feathering of the black ink andthe feathering of the color ink. The effect to avoid the color bleedaccording to the present invention will be explained in detail below.Two phenomena contribute to the effect to avoid the color bleedaccording to the present invention.

[0024] At first, the black ink and the color ink, which are dischargedfrom the printer and which arrive at the surface of the recording paper,make contact with each other on the recording paper while spreadingalong the surface of the recording paper respectively. Mixing takesplace between the black ink and the color ink at the boundary betweenthe inks at the contact portion. However, the inks are scarcely mixedwith each other, because of the presence of the difference in viscositytherebetween. The blurring between the inks at the boundary issuppressed to the minimum. Owing to the fact that the inks are scarcelymixed with each other, the width of the mixing area (hereinafterreferred to as “mixing width”) is suppressed to the minimum as well.

[0025] When the black ink and the color ink are mixed with each other,almost all portions are concealed by the black ink having the relativelydark color. In other words, almost all portions of the mixing width arerecognized as the black ink. According to this fact, if the position ofoccurrence of the mixing width is successfully adjusted so that theportion of the mixing width is included in the area which is to berecorded with the black ink, the mixed portion of the black ink and thecolor ink is recognized as the black ink. Therefore, the mixed portionis hardly recognized as the color bleed. Thus, it is possible topractically avoid the color bleed.

[0026] Secondly, the black ink and the color ink, which are dischargedfrom the printer and which arrive at the surface of the recording paper,make contact with each other on the recording paper while spreadingalong the surface of the recording paper respectively. Both of the blackink and the color ink, which have made contact with each other, intendto further spread along the recording paper surface. During thisprocess, the black ink is blocked by the spreading force of the colorink, and the black ink cannot spread any more, because the viscosity ofthe black ink is lower than that of the color ink. On the other hand,the color ink has the strong spreading force as compared with the blackink, because the viscosity of the color ink is relatively high.Accordingly, the color ink slightly spreads toward the black ink in sucha manner that the black ink is pushed back thereby. This phenomenon isalso a kind of color bleed. As a result of the experiment as describedlater on, it is effective that the viscosity ratio at 25° C. is adjustedto be not less than 1.3, in order to secure the slight spread. Owing tothis effect, the color ink, which has the relatively high viscosity,successfully spreads to the area of the black ink.

[0027] When the foregoing two phenomena are utilized in combination, themixing width of the black ink and the color ink is suppressed to theminimum. Further, owing to the fact that the color ink slightly spreadstoward the area of the black ink, the mixing width is included in thearea to be subjected to the recording with the black ink. Accordingly,the color bleed is avoided.

[0028] Other cases, which are different from those of the presentinvention, may be explained as follows.

[0029] If the viscosity of the black ink is higher than the viscosity ofthe color ink (viscosity ratio: less than 0.7) unlike the viscosityratio of the present invention, then the black ink spreads toward thecolor ink, and it is impossible to conceal the black ink with the colorink. Therefore, it is recognized that the color bleed occurs, and hencethe problem arises. The viscosity of the black ink is preferably 1.0 to8 (cps), and the viscosity of the color ink is preferably 2.0 to 11(cps).

[0030] If the viscosity of the black ink is approximately equivalent tothat of the color ink (viscosity ratio: less than 1±0.3), the mixingwidth is widened. Therefore, the blurring is conspicuous. Further, apart of the mixing width protrudes to the area in which the recording isto be originally performed with the color ink. For these reasons, it isrecognized that the color bleed occurs, and hence the problem arises.

[0031] Therefore, in order to suppress the movement of the black inktoward the color ink, it is necessary that the viscosity ratio at 25° C.is not less than 1.3.

[0032] Finally, if the surface tension of the black ink is not less than40 mN/m, and the surface tension of the color ink is less than 40 mN/m,then the color ink permeates into the paper simultaneously with thearrival at the recording paper because of the high permeability of thecolor ink, and the period of time, in which the color ink stays on therecording paper, is extremely short. Therefore, the color ink permeatesinto the recording paper before the spread of the black ink toward thecolor ink is suppressed by the difference in viscosity. The black ink,which makes contact with the color ink, spreads toward the color ink insuch a manner that the black ink is pulled by the color ink. As aresult, the blurring occurs.

[0033] Due to the phenomenon as described above, even when the surfacetension of the black ink is not less than 40 mN/m, if the surfacetension of the color ink is less than 40 mN/m, then the color bleed iseasily recognized, and hence the problem arises.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] An embodiment of the invention will be described in detail withreference to the following figures wherein:

[0035]FIG. 1 is a perspective view showing a color ink-jet printerhaving an ink cartridge which contains ink prepared in examples of theinvention;

[0036]FIG. 2 is a perspective view of a head unit, with its nozzlesfacing upward; and

[0037]FIG. 3 is a schematic diagram showing the ink jet print head and acontroller.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Examples of the present invention will be explained below.

[0039] In general, each of the black ink and the color ink to be usedfor the present invention is basically composed of the coloring agent,water, the water-soluble organic solvent, and the surfacetension-adjusting agent.

[0040] Usually, for example, dyes and pigments are used as the coloringagent.

[0041] Those usable as the dye include water-soluble dyes represented,for example, by direct dyes, acidic dyes, basic dyes, and reactive dyes.Especially, those, which are preferred for the ink for the ink-jetrecording method and which satisfy the required performance such asvividness, water-solubility, stability, and light resistance, include,for example, C.I. Direct Black 17, 19, 32, 51, 71, 108, 146, 154, 168;C.I. Direct Blue 6, 22, 25, 71, 86, 90, 106, 199; C.I. Direct Red 1, 4,17, 28, 83, 227; C.I. Direct Yellow 12, 24, 26, 86, 98, 132, 142; C.I.Direct Orange 34, 39, 44, 46, 60; C.I. Direct Violet 47, 48; C.I. DirectBrown 109; C.I. Direct Green 59; C.I. Acid Black 2, 7, 24, 26, 31, 52,63, 112, 118; C.I. Acid Blue 9, 22, 40, 59, 93, 102, 104, 113, 117, 120,167, 229, 234; C.I. Acid Red 1, 6, 32, 37, 51, 52, 80, 85, 87, 92, 94,115, 181, 256, 289, 315, 317; C.I. Acid Yellow 11, 17, 23, 25, 29, 42,61, 71; C.I. Acid Orange 7, 19; C.I. Acid Violet 49; C.I. Basic Black 2;C.I. Basic Blue 1, 3, 5, 7, 9, 24, 25, 26, 28, 29; C.I. Basic Red 1, 2,9, 12, 13, 14, 37; C.I. Basic Violet 7, 14, 27; and C.I. Food Black 1,2.

[0042] The exemplary dyes described above are especially preferred forthe water base ink for ink-jet recording of the present invention.However, the present invention is not limited to these dyes.

[0043] In addition to the carbon black, many inorganic pigments andorganic pigments may be used as the pigment. The pigment includes, forexample, azo pigment such as azo lake, insoluble azo pigment, condensedazo pigment, and chelate azo pigment; polycyclic pigment such asphthalocyanine pigment, perylene and perynone pigments, anthraquinonepigment, quinacridone pigment, dioxazine pigment, thioindigo pigment,isoindolinone pigment, and quinophthalone pigment; dye lake such asbasic dye type lake and acidic dye type lake; organic pigment such asnitro pigment, nitroso pigment, and aniline black daylight fluorescentpigment; and inorganic pigment such as titanium oxide, iron oxide-basedpigment, and carbon black-based pigment. Other pigments are also usableprovided that they are dispersible in the aqueous phase. Further, forexample, it is also possible to use those obtained by surface-treatingthe pigment as described above, for example, with a surfactant or apolymer dispersing agent, such as graft carbon.

[0044] The exemplary pigments described above are especially preferredfor the water base ink for ink-jet recording of the present invention.However, the present invention is not limited to these pigments.

[0045] When the pigment is used as the coloring agent of the presentinvention, the pigment is subjected to a dispersing treatment togetherwith an appropriate dispersing agent, a solvent, pure water, andoptionally other additives in accordance with the conventionally knownmethod.

[0046] Polymer dispersing agents and surfactants, which are described,for example, in Japanese Patent Application Laid-open No. 62-101672, maybe used as the dispersing agent. The polymer dispersing agent includes,for example, protein such as gelatin and albumin; natural rubber such asgum arabic and gum traganth; glucoside such as saponin; cellulosederivative such as methyl cellulose, carboxy cellulose, andhydroxymethyl cellulose; natural polymer such as lignosulfonate andshellac; anionic polymer such as salt of polyacrylic acid, salt ofstyrene-acrylic acid copolymer, salt of vinylnaphthalene-acrylic acidcopolymer, salt of styrene-maleic acid copolymer, salt ofvinylnaphthalene-maleic acid copolymer, and sodium salt and phosphoricacid salt of β-naphthalenesulfonic acid-formalin condensate; andnonionic polymer such as polyvinyl alcohol, polyvinyl pyrrolidone, andpolyethylene glycol. The surfactant includes, for example, anionicsurfactant such as higher alcohol sulfuric acid ester salt, liquid fattyoil sulfuric acid ester salt, and alkylarylsulfonic acid salt; andnonionic surfactant such as polyoxyethylene alky ether, polyoxyethylenealkyl ester, sorbitan alkyl ester, and polyoxyethylene sorbitan alkylester. One of the dispersing agents as described above may beappropriately selected and used singly, or two or more of the dispersingagents may be appropriately selected and used in combination. Ingeneral, it is desirable that the dispersing agent is used in an amountof 0.01 to 20% by weight with respect to the total amount of the ink.

[0047] On the other hand, any general dispersing machine may be used asthe dispersing machine to be used to disperse the pigment. However, thedispersing machine includes, for example, ball mills, roll mills, andsand mills. Especially, it is preferable to use a high speed type sandmill.

[0048] The dye and the pigment may be used singly respectively.Alternatively, two or more dyes, two or more pigments, or two or moredyes and pigments may be mixed and used.

[0049] The dye and/or the pigment is generally used in a ratio of 0.1 to20% by weight with respect to the water base ink for ink-jet recordingof the present invention. The dye and/or the pigment is desirably usedwithin a range of 0.3 to 15% by weight, and more desirably within arange of 0.5 to 10% by weight.

[0050] Usually, as for the water, it is preferable to use water havinghigh purity such as ion exchange water and distilled water, rather thanordinary water. In this case, the content of water is desirably not lessthan 20% by weight with respect to the total weight of the ink in orderthat the viscosity is maintained to be low so that application may bemade to the ink-jet recording based on a variety of methods, and thechange of the viscosity, which would be otherwise caused by thetemperature, is suppressed.

[0051] Usually, the water-soluble organic solvent is principally used inorder to avoid the drying-up and the occurrence of deposition from theink at the tip of the ink-jet head. Therefore, it is desirable to selectthe solvent which has low volatility and which has high solubility forthe dye. The water-soluble organic solvent as described above mayinclude, for example, polyvalent alcohols such as ethylene glycol,diethylene glycol, triethylene glycol, polyethylene glycol, propyleneglycol, dipropylene glycol, tripropylene glycol, polypropylene glycol,1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, glycerol,1,2,6-hexanetriol, 1,2,4-butanetriol, and 1,2,3-butanetriol,1,2,3-pentanetriol; nitrogen-containing heterocyclic compounds such asN-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 2-pyrrolidone,1,3-dimethylimidazolidinone, and ε-caprolactam; amides such asformamide, N-methylformamide, and N,N-dimethylformamide; amines such asmonoethanolamine, diethanolamine, triethanolamine, monoethylamine,diethylamine, and triethylamine; and sulfur-containing compounds such asdimethylsulfoxide, sulfolane, and thiodiethanol. The water-solubleorganic solvent may be used singly. Alternatively, two or more of thewater-soluble organic solvents may be used in a mixed manner. Those ofthe polyvalent alcohol alkyl ethers as described later on, which havethe vapor pressure at 20° C. of less than 0.01 mmHg, may be also used asthe water-soluble organic solvent.

[0052] The content of the water-soluble organic solvent in the ink is 5to 70% by weight, preferably 7 to 50% by weight, and more preferably 10to 40% by weight with respect to the total amount of the ink. If thecontent is less than 5% by weight, the moistening action is insufficientto cause, for example, problems of deposition and drying-up. If thecontent exceeds 40% by weight, the viscosity of the ink is unnecessarilyincreased. For example, the following problems arise. That is, the inkcannot be discharged, and the ink is dried extremely slowly on therecording paper.

[0053] Usually, the polyvalent alcohol alkyl ether or the surfactant isused as the surface tension-adjusting agent.

[0054] The polyvalent alcohol alkyl ether includes, for example,diethylene glycol monomethyl ether, diethylene glycol monobutyl ether,diethylene glycol monoisobutyl ether, diethylene glycol dimethyl ether,diethylene glycol dibutyl ether, dipropylene glycol monomethyl ether,dipropylene glycol monopropyl ether, dipropylene glycol monoisopropylether, dipropylene glycol monobutyl ether, dipropylene glycol dimethylether, dipropylene glycol dipropyl ether, dipropylene glycol dibutylether, triethylene glycol monomethyl ether, triethylene glycol monobutylether, tripropylene glycol monomethyl ether, tripropylene glycolmonobutyl ether, triethylene glycol dimethyl ether, triethylene glycoldibutyl ether, tripropylene glycol dimethyl ether, and tripropyleneglycol dibutyl ether.

[0055] Those of the polyvalent alcohol alkyl ethers described above,which have the vapor pressure at 20° C. of less than 0.01 mmHg, may bealso used as the water-soluble organic solvent.

[0056] The content of the polyvalent alcohol alkyl ether in the ink ispreferably 0.1 to 40% by weight with respect to the total amount of theink. If the content is less than 0.1% by weight, then the ink permeatesinto the recording paper at a slow speed, and any problem arisesconcerning the drying time and the color bleed. On the other hand, ifthe content exceeds 40% by weight, the ink intensely permeates into therecording paper. As a result, the ink arrives at the back of therecording paper, and any problem arises concerning the feathering aswell. Those usable as the surfactant include, for example, anionicsurfactants such as fatty acid salts and alkyl sulfuric acid estersalts; nonionic surfactants such as polyoxyethylene alkyl ether andpolyoxyethylene phenyl ether; acetylene glycol-based nonionicsurfactants such as Olfine E1004, E1010 (both trade names, produced byNissin Chemical Industry Co., Ltd.), and Surfynol 61, 82, 104, 440, 465,485 (all trade names, produced by Air Products and Chemicals); cationicsurfactants, and amphoteric surfactants. It is preferable that thecontent of the surfactant to serve as the permeating agent in the ink iswithin a range of 0.1 to 5% by weight.

[0057] Further, in order to control the drying performance and thepermeation of the ink into the recording paper, it is also possible touse monovalent alcohol such as ethanol and isopropyl alcohol.

[0058] The ink composition for ink-jet recording of the presentinvention is basically constructed as described above. Additionally, forexample, it is possible to add other conventionally known dispersingagents, viscosity-adjusting agents, pH-adjusting agents, dye-dissolvingagents, and antiseptic/fungicidal agents, if necessary.

[0059] When the ink, which is used for the ink-jet recording method ofthe type to electrically charge the recording liquid, is formulated, aspecific resistance-adjusting agent including, for example, inorganicsalts such as lithium chloride, ammonium chloride, and sodium chlorideis added.

[0060] When the ink of the present invention is applied to the ink-jetrecording method of the type in which the ink is discharged inaccordance with the action of the thermal energy, values of thermalphysical properties (for example, those of the specific heat, thecoefficient of thermal expansion, and the coefficient of thermalconductivity) are adjusted in some cases.

[0061] In the water base color ink set for ink-jet recording of thepresent invention obtained as described above, the problems involved inthe conventional technique are sufficiently solved. The feathering andthe color bleed are reduced in the ink-jet recording method. It ispossible to provide the vivid color recording.

[0062] Ink compositions according to the present invention will bedescribed as Examples below. Numerical values are expressed by % byweight. In respective Examples and Comparative Examples described below,the numerical values of the ink compositions are expressed by % byweight. The surface tension, the viscosity, and the viscosity ratio arethose obtained at 25° C.

EXAMPLE 1

[0063] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Example 1 are shown in Table 1. TABLE 1Example 1 Black Yellow Pure water 51.7 60.3 CAB-O-JET 300 black (carbonblack 33.3 — pigment, produced by Cabot) IJY214H (C. I. Direct Yellow86, produced — 2.7 by Daiwa Kasei Co., Ltd.) Triethylene glycolmonobutyl ether 3 3 Glycerol 12 24 Viscosity (cps) 2.0 3.0 Viscosityratio 1.5 Surface tension (mN/m) 45 46

[0064] The inks were prepared, in which the surface tension at 25° C.was not less than 40 mN/m, and the viscosity ratio between the black inkand the color ink (viscosity of the color ink/viscosity of the blackink) was not less than 1.3.

EXAMPLE 2

[0065] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Example 2 are shown in Table 2. TABLE 2Example 2 Black Yellow Pure water 46.7 60.3 CAB-O-JET 300 black (carbonblack 33.3 — pigment, produced by Cabot) IJY214H (C. I. Direct Yellow86, produced — 2.7 by Daiwa Kasei Co., Ltd.) Triethylene glycolmonobutyl ether 3 3 Glycerol 17 34 Viscosity (cps) 2.3 3.0 Viscosityratio 1.3 Surface tension (mN/m) 45 46

[0066] The inks were prepared, in which the composition of the yellowink of Example 1 was not changed, the viscosity of the black ink wasincreased by adjusting the amount of glycerol in the black ink, and theviscosity ratio was not less than 1.3.

EXAMPLES 3 and 4

[0067] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 3 and 4 are shown in Tables 3and 4. TABLE 3 Example 3 Black Yellow Pure water 79.3 62.3 MA-100(carbon black pigment, produced by 5 — Mitsubishi Chemical Corporation)Dysperbyk (anionic dispersing agent, 0.7 — produced by BYK Chemie)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol dimethyl ether 5 5 Glycerol 10 30 Viscosity(cps) 1.9 3.0 Viscosity ratio 1.6 Surface tension (mN/m) 55 54

[0068] TABLE 4 Example 4 Black Yellow Pure water 76.3 62.3 MA-100(carbon black pigment, produced by 5 — Mitsubishi Chemical Corporation)Dysperbyk (anionic dispersing agent, 0.7 — produced by BYK Chemie)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol dimethyl ether 5 5 Glycerol 13 30 Viscosity(cps) 2.3  3.0 Viscosity ratio 1.3 Surface tension (mN/m) 55 54

[0069] At first, a pigment mill base to be used for the black ink wasprepared in accordance with the following procedure.

[0070] Seventy-three (73) % by weight of pure water is added to 10% byweight of glycerol and 2% by weight of the dispersing agent, followed byperforming agitation. Fifteen (15) % by weight of the carbon black isadded thereto so that the total weight of the mill base is 100% byweight, followed by being dispersed with a bead mill for further 2hours. After completing the dispersing treatment with the bead mill,deaeration and filtration are performed to prepare the mill base. Themill base is used as the material for the black ink. The amounts ofglycerol and pure water are regulated in order to obtain the designedink composition.

[0071] In Examples 3 and 4, the inks were prepared, in which the speciesof the pigment was changed for the black ink as compared with Examples 1and 2, the species of the permeating agent was changed for the black inkand the yellow ink, and the viscosity ratio was not less than 1.3. Thecomposition of the yellow ink in Example 3 was the same as that inExample 4.

EXAMPLES 5 and 6

[0072] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 5 and 6 are shown in Tables 5and 6. TABLE 5 Example 5 Black Yellow Pure water 79.8 61.8 MA-100(carbon black pigment, produced by 5 — Mitsubishi Chemical Corporation)Dysperbyk (anionic dispersing agent, 0.7 — produced by BYK Chemie)IJY205H (C. I. A. Y. 23, produced by — 2.7 Daiwa Kasei Co., Ltd.)Triethylene glycol monobutyl ether 0.5 0.5 Glycerol 14 35 Viscosity(cps) 2.0 3.0 Viscosity ratio 1.5 Surface tension (mN/m) 59 58

[0073] TABLE 6 Example 6 Black Yellow Pure water 75.8 61.8 MA-100(carbon black pigment, produced by 5 — Mitsubishi Chemical Corporation)Dysperbyk (anionic dispersing agent, 0.7 — produced by BYK Chemie)IJY205H (C. I. A. Y. 23, produced by — 2.7 Daiwa Kasei Co., Ltd.)Triethylene glycol monobutyl ether 0.5 0.5 Glycerol 18 35 Viscosity(cps) 2.3 3.0 Viscosity ratio 1.3 Surface tension (mN/m) 59 58

[0074] A pigment mill base was prepared in accordance with the sameprocedure as that used in Examples 3 and 4, and the mill base was usedas the material for the black ink. Further, the inks were prepared, inwhich the surface tension was changed to about 60 mN/m by adjusting theamount of triethylene glycol-n-butyl ether in the yellow ink, and theviscosity ratio was not less than 1.3. The composition of the yellow inkin Example 5 was the same as that in Example 6.

EXAMPLES 7 and 8

[0075] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 7 and 8 are shown in Tables 7and 8. TABLE 7 Example 7 Black Yellow Pure water 57.3 26.3 Printex 150T(carbon black pigment, 5 — produced by Degussa) Dysperbyk (anionicdispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I. DirectYellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Triethylene glycolmonobutyl ether 3 3 Glycerol 34 68 Viscosity (cps) 6.0 10.2 Viscosityratio 1.7 Surface tension (mN/m) 48 47

[0076] TABLE 8 Example 8 Black Yellow Pure water 47.3 26.3 Printex 150T(carbon black pigment, 5 — produced by Degussa) Dysperbyk (anionicdispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I. DirectYellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Triethylene glycolmonobutyl ether 3 3 Glycerol 44 68 Viscosity (cps) 7.4 10.2 Viscosityratio 1.4 Surface tension (mN/m) 48 47

[0077] A pigment mill base was prepared in accordance with the sameprocedure as that used in Examples 3 and 4, and the mill base was usedas the material for the black ink. Further, the inks were prepared, inwhich the viscosity was changed to be within a relatively high viscosityregion, i.e., 6 to 10 cps by adjusting the amount of glycerol for theblack ink and the yellow ink, and the viscosity ratio was not less than1.3. The composition of the yellow ink in Example 7 was the same as thatin Example 8.

EXAMPLES 9 and 10

[0078] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 9 and 10 are shown in Tables 9and 10. TABLE 9 Example 9 Black Yellow Pure water 78.8 61 Monarch 880(carbon black pigment, 5 — produced by Cabot) Dysperbyk (anionicdispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I. DirectYellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Olfine E1010(surfactant, produced by 0.5  0.3 Nissin Chemical Industry Co., Ltd.)Glycerol 15 36 Viscosity (cps) 2.1 3.0 Viscosity ratio 1.4 Surfacetension (mN/m) 46 48

[0079] TABLE 10 Example 10 Black Yellow Pure water 75.8 61 Monarch 880(carbon black pigment, 5 — produced by Cabot) Dysperbyk (anionicdispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I. DirectYellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Olfine E1010(surfactant, produced by 0.5 0.3 Nissin Chemical Industry Co., Ltd.)Glycerol 18 36 Viscosity (cps) 2.3 3.0 Viscosity ratio 1.3 Surfacetension (mN/m) 46 48

[0080] A pigment mill base was prepared in accordance with the sameprocedure as that used in Examples 3 and 4, and the mill base was usedas the material for the black ink. Further, the inks were prepared, inwhich the surface tension was adjusted to 45 mN/m with the surfactantwithout using any permeating agent for the black ink and the yellow ink,and the viscosity ratio was not less than 1.3. The composition of theyellow ink in Example 9 was the same as that in Example 10.

EXAMPLES 11 and 12

[0081] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 11 and 12 are shown in Tables11 and 12. TABLE 11 Example 11 Black Yellow Pure water 51.7 67.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 3 — Triethylene glycol dimethylether — 30 Glycerol 12 — Viscosity (cps) 2.0 3.0 Viscosity ratio 1.5Surface tension (mN/m) 45 46

[0082] TABLE 12 Example 12 Black Yellow Pure water 44.7 67.3 CAB-O-JET300 black (carbon black 33.3 — pigment, produced by Cabot) IJY214H (C.I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co., Ltd.)Triethylene glycol monobutyl ether 3 — Triethylene glycol dimethyl ether— 30 Glycerol 19 — Viscosity (cps) 2.3 3.0 Viscosity ratio 1.3 Surfacetension (mN/m) 45 46

[0083] The inks were prepared, in which the same black ink as that usedin Examples 1 and 2 was used, the species of the permeating agent usedin the yellow ink was changed, and the viscosity ratio was not less than1.3. The composition of the yellow ink in Example 11 was the same asthat in Example 12.

EXAMPLES 13 and 14

[0084] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 13 and 14 are shown in Tables13 and 14. TABLE 13 Example 13 Black Magenta Pure water 51.7 57.5CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot) C.I. Direct Red 227 — 2.5 Triethylene glycol monobutyl ether 3 3 Glycerol12 37 Viscosity (cps) 2.0 3.0 Viscosity ratio 1.5 Surface tension (mN/m)45 46

[0085] TABLE 14 Example 14 Black Magenta Pure water 46.7 57.5 CAB-O-JET300 black (carbon black 33.3 — pigment, produced by Cabot) C. I. DirectRed 227 — 2.5 Triethylene glycol monobutyl ether 3 3 Glycerol 17 37Viscosity (cps) 2.3 3.0 Viscosity ratio 1.3 Surface tension (mN/m) 45 46

[0086] The inks were prepared, in which the same black ink as that usedin Examples 1 and 2 was used, the magenta ink was the color ink, and theviscosity ratio was not less than 1.3. The composition of the magentaink in Example 13 was the same as that in Example 14.

EXAMPLES 15 and 16

[0087] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Examples 15 and 16 are shown in Tables15 and 16. TABLE 15 Example 15 Black Cyan Pure water 51.7 58.5 CAB-O-JET300 black (carbon black 33.3 — pigment, produced by Cabot) C. I. DirectBlue 199 — 2.5 Triethylene glycol monobutyl ether 3 3 Glycerol 12 36Viscosity (cps) 2.0 3.0 Viscosity ratio 1.5 Surface tension (mN/m) 45 46

[0088] TABLE 16 Example 16 Black Cyan Pure water 46.7 58.5 CAB-O-JET 300black (carbon black 33.3 — pigment, produced by Cabot) C. I. Direct Blue199 — 2.5 Triethylene glycol monobutyl ether 3 3 Glycerol 17 36Viscosity (cps) 2.3 3.0 Viscosity ratio 1.3 Surface tension (mN/m) 45 46

[0089] The inks were prepared, in which the same black ink as that usedin Examples 1 and 2 was used, the cyan ink was the color ink, and theviscosity ratio was not less than 1.3. The composition of the cyan inkin Example 15 was the same as that in Example 16.

COMPARATIVE EXAMPLES 1, 2, and 3

[0090] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 1, 2, and 3 areshown in Tables 17, 18, and 19. TABLE 17 Comparative Example 1 BlackYellow Pure water 38.7 60.3 CAB-O-JET 300 black (carbon black 33.3 —pigment, produced by Cabot) IJY214H (C. I. Direct Yellow 86, produced —2.7 by Daiwa Kasei Co., Ltd.) Triethylene glycol monobutyl ether 3 3Glycerol 25 34 Viscosity (cps) 3.1 3.0 Viscosity ratio 1.0 Surfacetension (mN/m) 45 46

[0091] TABLE 18 Comparative Example 2 Black Yellow Pure water 43.7 60.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 3 3 Glycerol 20 34 Viscosity(cps) 2.7 3.0 Viscosity ratio 1.1 Surface tension (mN/m) 45 46

[0092] TABLE 19 Comparative Example 3 Black Yellow Pure water 38.7 70.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 3 3 Glycerol 25 24 Viscosity(cps) 3.1 2.0 Viscosity ratio 0.6 Surface tension (mN/m) 45 46

[0093] The inks were prepared, in which the composition of the yellowink in Examples 1 and 2 was not changed, the viscosity of the black inkwas increased by adjusting the amount of glycerol in the black ink, andthe viscosity ratio was less than 1.3.

COMPARATIVE EXAMPLES 4 and 5

[0094] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 4 and 5 are shownin Tables 20 and 21. TABLE 20 Comparative Example 4 Black Yellow Purewater 51.7 62.3 CAB-O-JET 300 black (carbon black 33.3 — pigment,produced by Cabot) IJY214H (C. I. Direct Yellow 86, produced — 2.7 byDaiwa Kasei Co., Ltd.) Triethylene glycol monobutyl ether 15 15 Glycerol— 20 Viscosity (cps) 2.0 3.2 Viscosity ratio 1.6 Surface tension (mN/m)35 35

[0095] TABLE 21 Comparative Example 5 Black Yellow Pure water 39.7 62.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 15 15 Glycerol 12 20 Viscosity(cps) 3.0 3.2 Viscosity ratio 1.1 Surface tension (mN/m) 36 35

[0096] The inks were prepared, in which the surface tension was 35 mN/mby adjusting the amount of triethylene glycol monobutyl ether withrespect to the compositions of the black ink and the yellow ink inExamples 1 and 2, and the viscosity ratio was not less than 1.3(Comparative Example 4) or less than 1.3 (Comparative Example 5) byadjusting the amount of glycerol in the black ink.

COMPARATIVE EXAMPLE 6

[0097] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Example 6 are shown in Table22. TABLE 22 Comparative Example 6 Black Yellow Pure water 46.7 62.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 3 15 Glycerol 17 20 Viscosity(cps) 2.3 3.2 Viscosity ratio 1.4 Surface tension (mN/m) 45 35

[0098] The inks were prepared, in which the same black ink as that usedin Example 1 was used, only the surface tension of the yellow ink was 35mN/m by adjusting the amount of triethylene glycol monobutyl ether, andthe viscosity ratio was not less than 1.3.

COMPARATIVE EXAMPLES 7 and 8

[0099] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 7 and 8 are shownin Tables 23 and 24. TABLE 23 Comparative Example 7 Black Yellow Purewater 68.3 62.3 MA-100 (carbon black pigment, produced by 5 — MitsubishiChemical Corporation) Dysperbyk (anionic dispersing agent, 0.7 —produced by BYK Chemie) IJY214H (C. I. Direct Yellow 86, produced — 2.7by Daiwa Kasei Co., Ltd.) Triethylene glycol dimethyl ether 5 5 Glycerol21 30 Viscosity (cps) 3.0 3.0 Viscosity ratio 1.0 Surface tension (mN/m)55 54

[0100] TABLE 24 Comparative Example 8 Black Yellow Pure water 71.3 62.3MA-100 (carbon black pigment, produced by 5 — Mitsubishi ChemicalCorporation) Dysperbyk (anionic dispersing agent, 0.7 — produced by BYKChemie) IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa KaseiCo., Ltd.) Triethylene glycol dimethyl ether 5 5 Glycerol 18 30Viscosity (cps) 2.7 3.0 Viscosity ratio 1.1 Surface tension (mN/m) 55 54

[0101] The inks were prepared, in which the composition of the yellowink in Examples 3 and 4 was not changed, the viscosity of the black inkwas increased by adjusting the amount of glycerol in the black ink, andthe viscosity ratio was less than 1.3.

COMPARATIVE EXAMPLES 9 and 10

[0102] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 9 and 10 are shownin Tables 25 and 26. TABLE 25 Comparative Example 9 Black Yellow Purewater 67.8 61.8 MA-100 (carbon black pigment, produced by 5 — MitsubishiChemical Corporation) Dysperbyk (anionic dispersing agent, 0.7 —produced by BYK Chemie) IJY205H (C. I. A. Y. 23, produced by — 2.7 DaiwaKasei Co., Ltd.) Triethylene glycol monobutyl ether 0.5 0.5 Glycerol 2635 Viscosity (cps) 3.0 3.0 Viscosity ratio 1.0 Surface tension (mN/m) 5958

[0103] TABLE 26 Comparative Example 10 Black Yellow Pure water 69.8 61.8MA-100 (carbon black pigment, produced by 5 — Mitsubishi ChemicalCorporation) Dysperbyk (anionic dispersing agent, 0.7 — produced by BYKChemie) IJY205H (C. I. A. Y. 23, produced by — 2.7 Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 0.5 0.5 Glycerol 24 35Viscosity (cps) 2.7 3.0 Viscosity ratio 1.1 Surface tension (mN/m) 59 58

[0104] The inks were prepared, in which the composition of the yellowink in Examples 5 and 6 was not changed, the viscosity of the black inkwas increased by adjusting the amount of glycerol in the black ink, andthe viscosity ratio was less than 1.3.

COMPARATIVE EXAMPLES 11 and 12

[0105] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 11 and 12 are shownin Tables 27 and 28. TABLE 27 Comparative Example 11 Black Yellow Purewater 47.3 32.3 Printex 150T (carbon black pigment, 5 — produced byDegussa) Dysperbyk (anionic dispersing agent, 0.7 — produced by BYKChemie) IJY306H (C. I. Direct Yellow 132, — 2.7 produced by Daiwa KaseiCo., Ltd.) Triethylene glycol monobutyl ether 3 3 Glycerol 44 62Viscosity (cps) 7.4 7.4 Viscosity ratio 1.0 Surface tension (mN/m) 48 47

[0106] TABLE 28 Comparative Example 12 Black Yellow Pure water 47.3 29.3Printex 150T (carbon black pigment, 5 — produced by Degussa) Dysperbyk(anionic dispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I.Direct Yellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Triethyleneglycol monobutyl ether 3 3 Glycerol 44 65 Viscosity (cps) 7.4 8.2Viscosity ratio 1.1 Surface tension (mN/m) 48 46

[0107] The inks were prepared, in which the composition of the black inkin Example 8 was not changed, the viscosity of the yellow ink waslowered by adjusting the amount of glycerol in the yellow ink whilemaintaining the relatively high viscosity region of the viscosity of 6to 10 cps, and the viscosity ratio was less than 1.3.

COMPARATIVE EXAMPLES 13 and 14

[0108] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 13 and 14 are shownin Tables 29 and 30. TABLE 29 Comparative Example 13 Black Yellow Purewater 68.8 61 Monarch 880 (carbon black pigment, 5 — produced by Cabot)Dysperbyk (anionic dispersing agent, 0.7 — produced by BYK Chemie)IJY306H (C. I. Direct Yellow 132, — 2.7 produced by Daiwa Kasei Co.,Ltd.) Olfine E1010 (surfactant, produced by 0.5 0.3 Nissin ChemicalIndustry Co., Ltd.) Glycerol 25 36 Viscosity (cps) 2.9 3.0 Viscosityratio 1.0 Surface tension (mN/m) 46 48

[0109] TABLE 30 Comparative Example 14 Black Yellow Pure water 70.8 61Monarch 880 (carbon black pigment, 5 — produced by Cabot) Dysperbyk(anionic dispersing agent, 0.7 — produced by BYK Chemie) IJY306H (C. I.Direct Yellow 132, — 2.7 produced by Daiwa Kasei Co., Ltd.) Olfine E1010(surfactant, produced by 0.5 0.3 Nissin Chemical Industry Co., Ltd.)Glycerol 23 36 Viscosity (cps) 2.7 3.0 Viscosity ratio 1.1 Surfacetension (mN/m) 47 48

[0110] The inks were prepared, in which the composition of the yellowink in Examples 9 and 10 was not changed, the viscosity of the black inkwas increased by adjusting the amount of glycerol in the black ink, andthe viscosity ratio was less than 1.3.

COMPARATIVE EXAMPLES 15 and 16

[0111] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 15 and 16 are shownin Tables 31 and 32. TABLE 31 Comparative Example 15 Black Yellow Purewater 63.7 67.3 CAB-O-JET 300 black (carbon black 33.3 — pigment,produced by Cabot) IJY214H (C. I. Direct Yellow 86, produced — 2.7 byDaiwa Kasei Co., Ltd.) Triethylene glycol monobutyl ether 3 —Triethylene glycol dimethyl ether — 30 Glycerol 25 — Viscosity (cps) 3.13.0 Viscosity ratio 1.0 Surface tension (mN/m) 45 46

[0112] TABLE 32 Comparative Example 16 Black Yellow Pure water 38.7 67.3CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot)IJY214H (C. I. Direct Yellow 86, produced — 2.7 by Daiwa Kasei Co.,Ltd.) Triethylene glycol monobutyl ether 3 — Triethylene glycol dimethylether — 30 Glycerol 25 — Viscosity (cps) 2.7 3.0 Viscosity ratio 1.1Surface tension (mN/m) 45 46

[0113] The inks were prepared, in which the composition of the yellowink in Examples 11 and 12 was not changed, the viscosity of the blackink was increased by adjusting the amount of glycerol in the black ink,and the viscosity ratio was less than 1.3 (the compositions of the blackinks were the same as those used in Comparative Examples 1 and 2).

COMPARATIVE EXAMPLES 17 and 18

[0114] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 17 and 18 are shownin Tables 33 and 34. TABLE 33 Comparative Example 17 Black Magenta Purewater 43.7 57.5 CAB-O-JET 300 black (carbon black 33.3 — pigment,produced by Cabot) C. I. Direct Red 227 — 2.5 Triethylene glycolmonobutyl ether 3 3 Glycerol 20 37 Viscosity (cps) 2.7 3.0 Viscosityratio 1.1 Surface tension (mN/m) 45 46

[0115] TABLE 34 Comparative Example 18 Black Magenta Pure water 38.757.5 CAB-O-JET 300 black (carbon black 33.3 — pigment, produced byCabot) C. I. Direct Red 227 — 2.5 Triethylene glycol monobutyl ether 3 3Glycerol 25 37 Viscosity (cps) 3.1 3.0 Viscosity ratio 1.0 Surfacetension (mN/m) 45 46

[0116] The inks were prepared, in which the composition of the magentaink in Examples 13 and 14 was not changed, the viscosity of the blackink was increased by adjusting the amount of glycerol in the black ink,and the viscosity ratio was less than 1.3 (the compositions of the blackinks were the same as those used in Comparative Examples 1 and 2).

COMPARATIVE EXAMPLES 19 and 20

[0117] The ink compositions, the surface tensions, the viscosities, andthe viscosity ratios of inks of Comparative Examples 19 and 20 are shownin Tables 35 and 36. TABLE 35 Comparative Example 19 Black Cyan Purewater 43.7 58.5 CAB-O-JET 300 black (carbon black 33.3 — pigment,produced by Cabot) C. I. Direct Blue 199 — 2.5 Triethylene glycolmonobutyl ether 3 3 Glycerol 20 36 Viscosity (cps) 2.7 3.0 Viscosityratio 1.1 Surface tension (mN/m) 45 46

[0118] TABLE 36 Comparative Example 20 Black Cyan Pure water 38.7 58.5CAB-O-JET 300 black (carbon black 33.3 — pigment, produced by Cabot) C.I. Direct Blue 199 — 2.5 Triethylene glycol monobutyl ether 3 3 Glycerol25 36 Viscosity (cps) 3.1 3.0 Viscosity ratio 1.0 Surface tension (mN/m)45 46

[0119] The inks were prepared, in which the composition of the cyan inkin Examples 15 and 16 was not changed, the viscosity of the black inkwas increased by adjusting the amount of glycerol in the black ink, andthe viscosity ratio was less than 1.3 (the compositions of the blackinks were the same as those used in Comparative Examples 1 and 2).

[0120] The respective materials were sufficiently mixed and agitated forthe ink compositions of Examples 1 to 16 and Comparative Examples 1 to20, followed by performing filtration with a membrane filter of 0.8 μmto use the obtained inks for the evaluation of recording.

[0121] The recording was performed with the black inks and the yellowinks by using a recording apparatus having a multi-head of the on-demandtype (discharge orifice diameter: 35 μm, resistance value of heatingresistor: 150 Ω, driving voltage: 30 V, frequency: 2 KHz) for performingthe recording by generating droplets by applying the thermal energy tothe ink in the recording head, and a recording apparatus having amulti-head of the on-demand type (discharge orifice diameter: 40 μm,driving voltage: 30 V, frequency: 10 KHz) for performing the recordingby generating droplets by applying the pressure to the ink in therecording head by means of the vibration of a piezoelectric element. Therecording sample was composed of a portion which included only singlecolor letters without any background, and a portion in which colors werecombined so that the letter color and the background color were formedby two color inks having different colors respectively. The evaluationwas directed to the distinction of letters and the disturbance of linesdue to any blurring of the ink at the single color portion, and thedistinction of letters and the blurring at the boundary at which thecolors might be mixed with each other. As for the size of the recordedletters, the letter size was set to 11 point with Microsoft Word 97. Therecording was performed by using regular paper (Xerox 4200) andregenerated paper (NEW SPEED HERB-100, abbreviation: SH100, produced byTokai-Pulp & Paper Co., Ltd.) on which the feathering and the colorbleed generally tend to occur as compared with regular paper. Therecording was performed in the same manner as described above with thecomparative inks as well.

[0122] Table 37 shows results of the evaluation of the recording samplesobtained when the printing was performed on the regular paper (Xerox4200). TABLE 37 Results of Evaluation of Feathering and Color Bleed onXEROX 4200 Paper Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Feathering (blackink) ++ ++ ++ ++ ++ ++ Feathering (color ink) ++ ++ ++ ++ ++ ++ Colorbleed ++ ++ ++ ++ ++ ++ Overall evaluation ++ ++ ++ ++ ++ ++ Ex. 7 Ex. 8Ex. 9 Ex. 10 Ex. 11 Ex. 12 Feathering (black ink) ++ ++ ++ ++ ++ ++Feathering (color ink) ++ ++ ++ ++ ++ ++ Color bleed ++ ++ + + ++ ++Overall evaluation ++ ++ + + ++ ++ Ex.13 Ex. 14 Ex. 15 Ex. 16 Feathering(black ink) ++ ++ ++ ++ Feathering (color ink) ++ ++ ++ ++ Color bleed++ ++ ++ ++ Overall evaluation ++ ++ ++ ++ Comp. Comp. Comp. Comp. Comp.Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Feathering (black ink) ++ ++++ − − ++ Feathering (color ink) ++ ++ ++ ± ± ± Color bleed − − − + ± −Overall evaluation − − − − − − Comp. Comp. Comp. Comp. Comp. Comp. Ex. 7Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Feathering (black ink) ++ ++ ++ ++ ++++ Feathering (color ink) ++ ++ ++ ++ ± ± Color bleed − − − − − −Overall evaluation − − − − − − Comp. Comp. Comp. Comp. Comp. Comp. Ex.13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Feathering (black ink) ++ ++ ++ ++++ ++ Feathering (color ink) ++ ++ ++ ++ ++ ++ Color bleed − − − − − −Overall evaluation − − − − − − Comp. Comp. Ex. 19 Ex. 20 Feathering(black ink) ++ ++ Feathering (color ink) ++ ++ Color bleed − − Overallevaluation − −

[0123] Table 38 shows results of the evaluation of the recording samplesobtained when the printing was performed on the regenerated paper(SH100). TABLE 38 Results of Evaluation of Feathering and Color Bleed onSH100 Paper Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Feathering (blackink) + + ++ ++ ++ ++ Feathering (color ink) ++ ++ ++ ++ ++ ++ Colorbleed ++ ++ ++ ++ + + Overall evaluation + + ++ ++ + + Ex. 7 Ex. 8 Ex. 9Ex. 10 Ex. 11 Ex. 12 Feathering (black ink) + + + + + + Feathering(color ink) ++ ++ ++ ++ ++ ++ Color bleed + + ± ± ++ ++ Overallevaluation + + + + + + Ex. 13 Ex. 14 Ex. 15 Ex. 16 Feathering (blackink) + + + + Feathering (color ink) ++ ++ ++ ++ Color bleed ++ ++ ++ ++Overall evaluation + + + + Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex.2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Feathering (black ink) + + + − − + Feathering(color ink) ++ ++ ++ − − − Color bleed − − − ± − − Overall evaluation −− − − − − Comp. Comp. Comp. Comp. Comp. Comp. Ex. 7 Ex. 8 Ex. 9 Ex. 10Ex. 11 Ex. 12 Feathering (black ink) + + + + + + Feathering (color ink)++ ++ ++ ++ + + Color bleed − − − − − − Overall evaluation − − − − − −Comp. Comp. Comp. Comp. Comp. Comp. Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17Ex. 18 Feathering (black ink) + + + + + + Feathering (color ink) ++ ++++ ++ ++ ++ Color bleed − − − − − − Overall evaluation − − − − − − Comp.Comp. Ex. 19 Ex. 20 Feathering (black ink) + + Feathering (color ink) ++++ Color bleed − − Overall evaluation − −

[0124] The two types of the recording apparatuses were used to evaluatethe printing in the present invention. Similar results were obtainedwith both of them.

[0125] The method for evaluating the feathering for the recording sampleis described below. The evaluation criterion is as follows. ++: Thefeathering is scarcely observable, and the letters are vivid. +: Thefeathering slightly occurs, but the letters are sufficiently readable.±: The feathering clearly occurs, but the letters are readable. −: Thefeathering clearly occurs, and it is difficult to read the letters.

[0126] The method for evaluating the color bleed for the recordingsample is described below. The evaluation criterion is as follows. ++:The color bleed is scarcely observable, and the letters are equivalentlyvivid as compared with the letters with no background. +: The colorbleed slightly occurs as compared with the letters with no background,but the letters are sufficiently readable. ±: The color bleed clearlyoccurs as compared with the letters with no background, but the lettersare readable. −: The color bleed clearly occurs as compared with theletters with no background, and it is difficult to read the letters.

[0127] The method for the overall evaluation of the recording sample isdescribed below. The evaluation criterion is as follows. ++: Theevaluation of the feathering is ++, the evaluation of the color bleed is++, and the letters are vivid. +: The evaluation of the feathering is++, +, or ±, the evaluation of the color bleed is ++, +, or ±, and theletters are sufficiently readable. −: The evaluation of − is made in anyone of the evaluation of the feathering and the evaluation of the colorbleed, and it is difficult to read the letters in some portions.

[0128] The results of the evaluation of the printing are explainedbelow.

[0129] In Examples 1 and 2 and Comparative Examples 1, 2, and 3, thesurface tension at 25° C. was not less than 40 mN/m, the viscosity ofthe black ink was adjusted depending on the difference in amount ofglycerol in the black ink, and only the viscosity ratio between theblack ink and the color ink at 25° C. was changed. On this condition,the results of the overall evaluation of the printing on the regularpaper XEROX 4200 were as follows. That is, in Example 1, the viscosityratio was 1.5, and the evaluation result was ++. In Example 2, theviscosity ratio was 1.3, and the evaluation result was ++. InComparative Example 1, the viscosity ratio was 1.0, it was unsuccessfulto satisfy the color bleed, and the evaluation result was −. InComparative Example 2, the viscosity ratio was 1.1, and the evaluationresult was − in the same manner as in Comparative Example 1. InComparative Example 3, the viscosity ratio was 0.6, and the evaluationresult was − in the same manner as in Comparative Example 1. Further,the results of the overall evaluation of the printing on the regeneratedpaper SH100 were as follows. That is, in Example 1, the evaluationresult was +. In Example 2, the evaluation result was +. In ComparativeExample 1, it was unsuccessful to satisfy the color bleed in the samemanner as in the case of the regular paper XEROX 4200, and theevaluation result was −. In Comparative Example 2, the evaluation resultwas − in the same manner as in Comparative Example 1. In ComparativeExample 3, the evaluation result was − in the same manner as inComparative Example 1. Although the printing quality was slightlyinferior to that obtained in the case of the printing on the regularpaper. However, the similar results were successfully obtained.

[0130] In Comparative Examples 4 and 5, the surface tension at 25° C.was 35 mN/m by adjusting the amount of triethylene glycol monobutylether with respect to the compositions of the black ink and the yellowink in Examples 1 and 2, the viscosity of the black ink was adjusteddepending on the difference in amount of glycerol in the black ink, andthe viscosity ratio between the black ink and the color ink at 25° C.was changed. On this condition, the results of the overall evaluation ofthe printing on the regular paper XEROX 4200 were as follows. That is,in Example 1, the viscosity ratio was 1.5, and the evaluation result was++. In Example 2, the viscosity ratio was 1.3, and the evaluation resultwas ++. In Comparative Example 4, the viscosity ratio was 1.6, it wasunsuccessful to satisfy the feathering for both of the black ink and thecolor ink, and the evaluation result was −. In Comparative Example 5,the viscosity ratio was 1.1, and the evaluation result was − in the samemanner as in Comparative Example 4. Further, the results of the overallevaluation of the printing on the regenerated paper SH100 were asfollows. That is, in Example 1, the evaluation result was +. In Example2, the evaluation result was +. In Comparative Example 4, it wasunsuccessful to satisfy the feathering for both of the black ink and thecolor ink in the same manner as in the case of the regular paper XEROX4200, and the evaluation result was −. In Comparative Example 5, theevaluation result was − in the same manner as in Comparative Example 4.Although the printing quality was slightly inferior to that obtained inthe case of the printing on the regular paper XEROX 4200. However, thesimilar results were successfully obtained even in the case of theregenerated paper SH100.

[0131] According to the results described above, it is affirmed that thecondition, in which both of the requirements for the feathering and thecolor bleed are satisfied, is as follows. That is, the surface tensionat 25° C. is not less than 40 mN/m, and the viscosity ratio is not lessthan 1.3.

[0132] Further, the similar results were also obtained in other Examplesand Comparative Examples. According to the results described above, itis affirmed that the condition, in which both of the requirements forthe feathering and the color bleed of the black ink and the color inkare satisfied, is as follows. That is, the surface tension at 25° C. isnot less than 40 mN/m, and the viscosity ratio is not less than 1.3.

[0133] In respective Examples, in the case of the water base color inksets for ink-jet recording constructed according to the presentinvention, i.e., in the case of the water base color ink sets forink-jet recording in which both of the surface tensions at 25° C. of theblack ink and the color ink were not less than 40 mN/m, and theviscosity ratio between the black ink and the color ink was not lessthan 1.3, the satisfactory results for the feathering and the excellentresults for the color bleed were successfully obtained with the blackink and the color ink respectively. On the other hand, in ComparativeExamples constructed contrarily to the present invention, any problemarose in any one of the feathering and the color bleed of the black inkand the color ink, and it was impossible to obtain any satisfactoryprinting quality.

[0134] An embodiment of an ink jet printer as an ink-jet recordingapparatus in accordance with the invention will be described as belowwith reference to the accompanying drawings.

[0135] As shown in FIG. 1, a color ink jet printer 100 includes four inkcartridges (ink set) 61, each of which contains a respective color ofink, such as cyan, magenta, yellow and black ink, a head unit 63 havingan ink jet printer head 6 (hereinafter referred to as a head 6) forejecting ink onto a sheet 62, a carriage 64 on which the ink cartridges61 and the head unit 63 are mounted, a drive unit 65 that reciprocatesthe carriage 64 in a straight line, a platen roller 66 that extends in areciprocating direction of the carriage 64 and is disposed opposite tothe head 6, and a purge unit 67. As the black, cyan, magenta and yellowink, the ink prepared in the above examples can be used.

[0136] The drive unit 65 includes a carriage shaft 71, a guide plate 72,two pulleys 73 and 74, and an endless belt 75. The carriage shaft 71 isdisposed at a lower end portion of the carriage 64 and extends inparallel with the platen roller 66. The guide plate 72 is disposed at anupper end portion of the carriage 64 and extends in parallel with thecarriage shaft 71. The pulleys 73 and 74 are disposed at both endportions of the carriage shaft 71 and between the carriage shaft 71 andthe guide plate 72. The endless belt 75 is stretched between the pulleys73 and 74.

[0137] As the pulley 73 is rotated in normal and reverse directions by amotor, the carriage 64, connected to the endless belt 75, isreciprocated in the straight direction, along the carriage shaft 71 andthe guide plate 72, in accordance with the normal and reverse rotationof the pulley 73.

[0138] The sheet 62 is supplied from a sheet cassette (not shown)provided in the ink jet printer 100 and fed between the head 6 and theplaten roller 66 to perform predetermined printing by ink dropletsejected from the head 6. Then, the sheet 62 is discharged to theoutside. A sheet feeding mechanism and a sheet discharging mechanism areomitted from FIG. 1.

[0139] The purge unit 67 is provided on a side of the platen roller 66.The purge unit 67 is disposed to be opposed to the head 6 when the headunit 63 is located in a reset position. The purge unit 67 includes apurge cap 81, a pump 82, a cam 83, and a waste ink reservoir 84. Thepurge cap 81 contacts a nozzle surface to cover a plurality of nozzles(described later) formed in the head 6. When the head unit 63 is placedin the reset position, the nozzles in the head 6 are covered with thepurge cap 81 to inhale ink including air bubbles trapped in the head 6by the pump 82 and by the cam 83, thereby purging the head 6. Theinhaled ink is stored in the waste ink reservoir 84.

[0140] To prevent ink from drying, a cap 85 is provided to cover thenozzles 15 (FIG. 2) in the head 6 mounted on the carriage 64 when itreturns to the reset position after printing.

[0141] As shown in FIG. 2, the head unit 63 is mounted on the carriage64 that moves along the sheet 62 and has a substantially box shape withupper open structure. The head unit 63 has a cover plate 44 made of anelastic thin metallic plate. The cover plate 44 is fixed at the frontsurface of the head unit 63 and covers the head unit 63 when the head 6is removed. The head unit 63 also has a mounting portion 2 on which thefour ink cartridges 61 are detachably attached from above. Ink supplypaths 4 a, 4 b, 4 c, 4 d, each of which connects respective inkdischarge portions of each ink cartridge 61, communicate with a bottomof a bottom plate 5 of the head unit 63. Each of the ink supply paths 4a, 4 b, 4 c, 4 d is provided with a rubber packing 47 to intimatelycontact an ink supply hole 19 a.

[0142] The head 6 is constructed from four blocks that are arranged inparallel to each other. On the underside of the bottom plate 5, fourstepped supports 8 are formed to receive the respective blocks of thehead 6. In the bottom plate 5, a plurality of recesses 9 a, 9 b, whichare filled with an UV adhesive to bond the respective blocks of the head6, are formed to penetrate the bottom plate 5.

[0143]FIG. 3 is a sectional view showing one of the pressure chambers inthe head 6. A plurality of pressure chambers 16 are provided in the head6. The nozzles 15 communicating the respective pressure chambers 16 areprovided substantially in line in one surface of the head 6.

[0144] As shown in FIG. 3, the head 6 is constructed by the cavity plate10 and the piezoelectric actuator 20. The cavity plate 10 has the inksupply holes 19 a connected with the ink cartridge 61, the manifolds 12,the narrowed portions 16 d, the pressure chambers 16, the through holes17 and the nozzles 15, which communicate with each other. While the inksupply hole 19 a opens toward the ejecting direction of the nozzle 15 inFIG. 3 for convenience, the ink supply hole 19 a actually opens towardthe piezoelectric actuator 20.

[0145] A controller 3 provides a prestored driving pulse to thepiezoelectric actuator 20 by superimposing the driving pulse on a clocksignal. The driving pulse can be controlled with a technique disclosedin, for example, U.S. Pat. Nos. 6,312,089, 6,412,923 B1 and 6,760,959.Further, the detailed structure of the printer and controlling method ofthe head unit are also disclosed in these U.S. patents, a content ofwhich has been incorporated herein by reference.

[0146] The present invention is not limited to Examples describedherein, which can be carried out even when the material substances, theamounts thereof, and the preparation conditions to be used are changedwithin the ranges as described above.

[0147] As clarified from the facts explained above, the water base inkset for ink-jet recording comprising the black ink and the color ink ischaracterized in that both of the surface tensions at 25° C. of theblack ink and the color ink are not less than 40 mN/m, and the viscosityratio between the black ink and the color ink (viscosity of the colorink/viscosity of the black ink) is not less than 1.3. Therefore, thefeathering and the color bleed are simultaneously avoided for the blackink and the color ink, and it is possible to perform the vivid colorrecording.

What is claimed is:
 1. An ink set for ink-jet recording, comprising: ablack ink which has a surface tension of not less than 40 mN/m at 25°C.; and a color ink which has a surface tension of not less than 40 mN/mat 25° C., wherein: a viscosity ratio of the color ink with respect tothe black ink is not less than 1.3.
 2. The ink set according to claim 1,wherein the surface tensions of the color ink and the black ink are notless than 45 mN/m at 25° C.
 3. The ink set according to claim 1, whereinthe color ink has a viscosity of 2.0 to 11.0 cps, and the black ink hasa viscosity of 1.0 to 8.0 cps.
 4. The ink set according to claim 1,wherein each of the black ink and the color ink contains a coloringagent, water, and a water-soluble organic solvent.
 5. The ink setaccording to claim 4, wherein each of the black ink and the color inkfurther contains a surface tension-adjusting agent.
 6. The ink setaccording to claim 4, wherein the water-soluble organic solvent isglycerol.
 7. The ink set according to claim 1, wherein the color inkincludes an yellow ink, a magenta ink, and a cyan ink.
 8. The ink setaccording to claim 7, wherein the ink set is in a form of ink cartridge.9. An ink-jet recording apparatus comprising: an ink-jet head; an inktank which accommodates an ink to be supplied to the ink-jet head; andan ink set as defined in claim 1 which is accommodated in the ink tank.10. The ink-jet recording apparatus according to claim 9, wherein theink tank is in a form of ink cartridge.